It is not the lifetime and content of the VMs that matter. It is the physical machine on which the License Manager VM runs that matters. The licensing system as we are using it unfortunately depends on some aspects of the machine (ie CPU type) that are not virtualized. In the end, this ties the licensing system to the underlying physical host on which the License Manager VM runs. If you can't control your License Manager VM in the VM environment, then you don't know which physical host your VM is running on.
If you're running your License Manager VM in a cluster of identical physical hosts... no problem. If your License Manager VM doesn't move from its physical host... no problem. But if your License Manager VM is wandering about, running on non-identical physical machines... problem. The licenses will be invalidated when the License Manager VM runs on a different physical host.
If you can control your VM environment, you can mitigate the problem by 'pinning' the VM to run on the same physical host. This works well in most cases.
Remember: this is only a problem for the License Manager VM (serving the licenses). It is not an issue for the VMs simply running TestComplete or TestExecute (which aren't serving licenses). (And this is only a problem for the License Manager on VMs. The licensing system uses different criteria to judge machine integrity for physical machines.)